[uss_qualifier] add utility properties to QueryError

[Shastick]

A QueryError often wrap a single query, of which it is common to check the status code.

Additionally, when a check fails, we're interested in the wrapped queries' timestamps.

This PR adds a last_status_code and query_timestamps method to QueryError to make these use cases easier.

[mickmis]

I believe that the convention was that the "main" query is actually the first one, at least that what I did in some tests, and I don't believe I saw other examples of that (but there may be). Not great if we have two conventions.
IMO we would be better of introducing the convention (part of the class contract) that we have one single relevant query (then we can just have status_code e.g.), and offer the possibly of adding additional 'FYI' queries. I don't think that would break anything. WDYT?

[Shastick]

That makes sense, though I'm happy to say that the first query would be the 'main' one, and to reflect that in the various utility methods.

If this is properly documented (such as through a comment or well-named utility functions) I'd be fine with it.

I'll see if I can come up with something simple.

[mickmis]

Nice LGTM